Lecture vii. 



DEVELOPMENT OF CELLS. 



We have hitherto considered the cells in the condition in which they immediately 

 present themselves in the living parts of plants under the microscope. The organs, 

 however, at first themselves microscopically small, grow up later to a considerable 

 size. This gi-owth must necessarily also affect the cells of which the organs consist; 

 and thus, with increasing growth of the organs, the cells also would have to attain 

 a considerable size if, during the growth itself, a diminution and at the same time 

 multiplication of them did not take place by division. In general, the growth of 

 the organs of the plant, especially in its first stages, is associated with multiplication 

 of the existing cells ; and only in the final processes of growth, where the organs 

 attain their definitive form and size, does the multiplication of the cells in them cease, 

 and the cells themselves attain their ultimate form and size. 



Thus, to observe the formation of new cells we must not make use of organs 

 already fully grown; we must rather examine the growing points and the parts 

 which are becoming elongated. However, although there is no doubt that cell-divisions 

 are constantly taking place in the growing points, it is still very difficult to observe 

 this process there. On the other hand, it is relatively easy to see the multiplication of 

 the cells further distant from the growing points in the parts becoming elongated; 

 and moreover, as experience has shown, it is particularly in the development of the 

 organs of reproduction that the processes of cell-formation may be easily seen. 



Above all, it is to be premised that the multiplication of cells is always brought 

 about by the division of cells already existing ; and that new cells at no time shoot 

 forth like crystals out of a fluid. Each newly arising cell originates by division 

 of one already existing ; and very generally this division is a bipartition, i. e. two 

 daughter-cells arise from a mother-cell from the substance of the former becoming 

 divided into two equal halves. 



Before 1 enter more in detail into the processes which take place in the bipar- 

 tition of a cell, it will be advisable to consider the more obvious facts. 



Since the cells generally appear as chambers in the substance of the plant, their 

 new formation or division may also be represented, especially during growth, as a 

 progressive formation of chambers or compartments. Within the cells already 



